This invention relates to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein. In the process of electrostatographic printing, electrostatic latent charge patterns are formed on an insulating medium for the purpose of recording and reproducing the patterns in viewable form.
The field of electrostatographic printing includes both electrographic and electrophotographic printing. Electrophotographic printing employs a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent charge pattern thereon. Electrographic printing utilizes an insulating medium to form, without the aid of electromagnetic radiation, an electrostatic latent charge pattern. Development, which is the act of rendering an electrostatic pattern or image viewable, is employed in all of the aforementioned types of electrostatographic printing. An electrophotographic printing machine is described hereinafter as an illustrative embodiment of this process.
Electrophotographic printing charges a photoconductive surface to a substantially uniform potential to sensitize the surface. The charged photoconductive surface is, thereafter, exposed to a light image of the original document being reproduced. As a consequence of this exposure, the charge on the photoconductive surface is selectively dissipated in accordance with the light intensity reaching the surface. This creates an electrostatic latent image on the photoconductive surface corresponding to the original document.
Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. Typical developer mixes employed in the art generally comprise dyed or color thermoplastic powders, known as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles have the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface, the greater attractive force of the electrostatic latent image causes the particles to transfer from the carrier granules and adhere to the latent image. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
Many factors influence the quality of the developed image, the most significant factor being the uniformity with which the toner particles are deposited on the latent image. Heretofore, development systems have employed rotary impellers, fur brushes, bucket conveyors and magnetic brush systems to achieve the requisite uniformity in toner deposition. Magnetic brush systems achieve a high degree of uniform toner deposition, and are, therefore, used in numerous printing machines. In a magnetic brush system, a developer roll has a directional flux field which assists in bringing the magnetizable developer mix into contact with the latent image. The magnetic field causes the developer mix to form chain-like arrays similar to bristles of a brush. This brush of developer mix moves the latent image transferring toner particles from the carrier granules thereto. Thus, the developer roll serves to transport the developer mix from a sump in a housing to the latent image to render the latter visible. Various types of magnetic brush rolls have been developed. Exemplary of these is U.S. Pat. No. 3,040,704 issued to Bliss which describes a magnetic brush roll having a roughened exterior surface so as to facilitate the transportation of the developer mix. Many techniques have been employed to roughen the exterior surface of the developer roll. An example of one approach is disclosed in U.S. Pat. No. 3,246,629 issued to Shelffo. In this patent a flame spray is used to provide layer of irregular shaped particles which adhere to the exterior circumferential surface of the developer roll providing a randomly roughened surface. Other techniques for increasing the coupling between the developer roll and developer mix so as to improve the conveying function may be roughening the surface by knurling, shot peening, or by adhesively placing beads on sand on the outer surface. However, it is possible that this type of rough, abrasive surface does considerable mechanical damage to the developer mix at points where there is a speed mis-match between the roller and the developer mix. For example, at the feed or transfer point between the rollers and the developer mix or where the developer mix passes through a restricted orifice such as a metering blade or development point, the developer mix tends to wear. In addition, it has been found that the developer roller tends to become somewhat smoother with prolonged use. Thus, not only does the developer mix wear but the developer roller wears producing a shorter life for both the developer mix and developer roll. Moreover, the roundness of the developer roll is a critical tolerance. If the developer roll is not round, a strobing effect can be produced on the resultant copy. Thus, the roundness of the developer roll must be maintained within tolerance at all times.
Other approaches have been developed for providing a developer roll which does not induce wear on the developer mix while still having sufficient life. An example of this is described in U.S. Pat. No. 3,863,603 issued to Buckley et al.. A magnetic brush roller is described therein as having a resilient, roughened polyurethane coated on a metal tube. Similarly, U.S. Pat. No. 3,176,652 issued to Mott describes a magnetic brush apparatus having an elongated magnet held stationarily in a rotating shield. The shield may be plastic with the outer surface thereof roughened in a random or rectangular pattern. Finally, U.S. Pat. No. 3,563,734 issued to Shely discloses a developer powder applicator made from a roller coated with a conductive rubber or hardened conductive gelatin. The roller may be rotatable, hollow non-magnetic metal cylinder containing a stationary permanent magnet therein.
In experimentation, it has been found that a urethane coating may lack sufficient wear life to be suitable for use in an electrophotographic printing machine. Thus, the urethane coating may become smooth and slippery after several thousand copies have been made. For example, it has been found that the coefficient of friction off a urethane coating may change from 0.83 to 0.38 after three thousand copies. This is primarily due to the retention of toner particles in the urethane coating and the wear thereof.
Accordingly, it is the primary object of the present invention to improve the developer roll of a development system employed in an electrophotographic printing machine.